The Moon is generally considered to be dry. There are two reasons for that. The first is the generally accepted model for the formation of our moon is that something about the size of Mars collided with Earth and sent a huge amount of silica vapours into space at temperatures of about 10,000 degrees Centigrade (which is about twice as hot as the average surface of the sun) and much of that (some say about half) condensed and accreted into the Moon. Because the material was so hot and in a vacuum, all water should have been in the gas phase, and very little would condense so the Moon should be anhydrous deep in the interior. The fact its volcanic emissions have been considered to be dry is taken to support that conclusion. And thus with circular logic, it supports the concept that Earth formed by objects as large as Mars colliding and forming the planet.
The second is the rocks brought back by Apollo were considered to be anhydrous. That was because the accepted paradigm for the Moon formation required it to be dry. The actual rocks, on heating to 700 degrees Centigrade, were found to have about 160 ppm of water. On the basis that the accepted paradigm required them to be anhydrous it was assumed the rocks were contaminated with water from Earth. The fact that the deuterium levels of the hydrogen atoms in this water corresponded to solar hydrogen and not Earth’s water was ignored. That could not be contamination. Did that cause us to revise the paradigm? Heavens no. Uncomfortable facts that falsify the accepted theory have to be buried and ignored.
Recently, two scientific papers have concluded that the surface of the Moon contains water. Yay! If we go there, there is water to drink. Well, maybe. First, let’s look at how we know. The support is from infrared spectra, where a signal corresponding to the O-H bond stretching mode is seen. It has been known for some time that such signals have been detected on the Moon, but this does not mean there is water, since it could also arise from entities with, say, a Si-O-H group. Accordingly, it could come from space weathered rock, and in this context, signal strength increases towards the evening, which would happen if the rocks reacted with solar wind. The heating of rocks with these groups would give off water, so the Moon might still be technically dry but capable of providing water. Further examination of apatites brought back from Apollo suggested the interior could have water up to about 400 ppm.
How could the interior be wetter? That depends on how it formed. In my ebook, “Planetary Formation and Biogenesis” I surveyed the possibilities, and I favour the proposal outlined by Belbruno, E., Gott, J.R. 2005. Astron. J. 129: 1724–1745. Quite simply, Theia, the body that collided with Earth, formed at one of the Lagrange points. I favour L4. Such a body there would accrete by the same mechanism as Earth, which explains why it has the same isotopes, and while its orbit there is stable while it is small, as soon as it becomes big it gets dislodged. It would still collide with Earth, it would still get hot but need not vaporize. Being smaller, the interior may trap its water. There is evidence from element abundance that anything that would remain solid on the surface at about 1100 degrees Centigrade was not depleted, which means that is roughly the maximum temperature reached, and that would not vaporize silicates.
In one of the new papers, the signals from the surface have included the H-O-H bending frequency, which means water. Since it has not evaporated off into space it is probably embedded in rocks and may have originated from meteorites that crashed into the Moon, where they melted on impact and embedded the water they brought. There is also ice in certain polar craters that never see the sunlight, and above latitude 80 degrees, there are a number of such small craters.So, what does this mean for settlement? If the concentration is 5 ppm, to get 5 kg of water you would have to process a thousand tonne of rock, which would involve heating it to about seven hundred degrees Centigrade, holding it there, and not letting any water escape. The polar craters have ice up to a few per cent, but that ice also contains ammonia, hydrogen sulphide, and some other nasties, and since the craters never see sunlight the outside temperature is approximately two hundred degrees Centigrade below zero. You will see proposals that future space ships will use hydrogen and oxygen made from lunar water. That would require several thousand tonne of water, which would involve processing a very large amount of rock. It will always be easier to get water from the Sahara desert than the lunar surface, but it is there and could help maintain a settlement with careful water management.
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